The present invention relates to a copper-containing conductor composition, and more particularly to a composition for providing a thick film copper conductor having an improved solderability (wettability by solder), and a conductor element including a patterned conductor layer prepared from the composition.
Thick film conductors are widely used as a means for interconnecting components or parts in various electronic devices, including hybrid microcircuits. In general, conductors are required to have properties such as electric conductivity, solderability, solder leach resistance and adhesion to substrates or base materials. Thick film conductor compositions are composed of a conductive metal and an inorganic binder, which are in finely divided form and are dispersed in an organic medium, and optionally other additives. These ingredients are selected according to performance characteristics which are sought.
Thick film conductors using copper as conductive metal have been introduced at first from the viewpoint of cost reduction for thick film conductors containing as conductive metal an expensive noble metal such as gold, silver, platinum or palladium. Recently, attention is paid to characteristics that copper itself has, particularly to high frequency characteristics, thus various thick film copper conductor compositions have been proposed and applied to various fields. For instance, U.S. Pat. No. 2,993,815 to Treptow discloses a conductor composition for printed circuits containing 5 to 50% by weight of copper or copper oxide and 1% by weight of reduction-resistant glass frit, from which a copper conductive layer is formed by firing in two stages at a temperature of 500 to 1050.degree. C. U.S. Pat. No. 3,647,532 to Friedman discloses a composition containing copper, a glass frit and a lead borosilicate glass binder containing cadmium oxide. U.S. Pat. No. 3,988,647 to Bolon et al discloses a conductor composition containing copper particles, which have been treated to remove oxide from the surface, dispersed in a solventless polymeric binder. U.S. Pat. No. 4,070,518 to Hoffman discloses a conductor composition containing 85 to 97% by weight of copper powder and 3 to 15% by weight of a Cd- and Bi-free lead aluminoborate glass frit. U.S. Pat. No. 4,072,771 to Grier discloses a conductor composition containing Cu particles which have been preoxidized to form a CuO surface layer, the amount of CuO being 1 to 5% by weight of the total solids, and 1 to 10% by weight of, based on the total solids, a lead aluminoborate glass frit.
Also, U.S. Pat. No. 4,172,919 to Mitchell discloses a conductor composition containing 86 to 97% by weight of copper, 1 to 7% by weight of CuO, and 1 to 7% by weight of a glass frit containing at least 75% by weight of Bi.sub.2 O.sub.3. U.S. Pat. No. 4,322,316 to Provance discloses a conductor composition containing 7 to 27% by Weight of boron, 0 to 35% by weight of a glass frit and a residual amount of CuO. U.S. Pat. No. 4,323,483 to Rellick discloses a conductor composition comprising copper, copper oxide, lead oxide and bismuth oxide, which does not require the use of a glass frit. U.S. Pat. No. 4,521,329 to Siuta it al discloses a conductor composition containing a copper powder having an oxide coating, copper oxide and an inorganic binder having softening point of 300.degree. to 700.degree. C. Further, in published European Patent Application No. 0068167, McCormick et al disclose a conductor composition containing 65 to 80% by weight of Cu, 0 to 6% by weight of CuO and 3 to 8% by weight of Bi-free low softening glass.
Upon formation of copper conductors, the conductor compositions as mentioned above are applied to suitable substrates, and the applied compositions are fired. The firing is conducted in a nonoxidizing atmosphere, usually in a nitrogen atmosphere the oxygen content of which is from 1 to 10 p.p.m., for preventing oxidation of copper. However, it is known that the surface of copper conductor is gradually oxidized by a trace amount of oxygen remaining in the nitrogen atmosphere, thus resulting in deterioration of solderability. The deterioration in solderability is marked particularly when the firing is repeated. Therefore, for avoiding such problem it is necessary to shorten the firing time, or to conduct the firing in a very pure nitrogen atmosphere, or to decrease the number of firing operations, thus applications of conventional conductor compositions as mentioned above have been restricted.
In order to secure a good solderability even after firing repeated plural times, Siuta et al propose conductor compositions containing 0.2 to 5% by weight of metallic W, Mo, Re, alloys or mixtures thereof in U.S. Pat. Nos. 4,514,321 and 4,540,604. W, Mo and Re are used for the purpose of providing copper having a substantially oxide free surface by a reaction of W, Mo or Re with the oxide in the copper surface and oxygen present in a trace amount in the firing atmosphere. on the other hand, the copper particles used have a copper oxide coating. Since the copper oxide coating would deteriorate the soldarability, copper oxide must be reduced by the above metals such as W during the firing. Therefore, when the portion of these metals which are consumed to reduce copper oxide increases, in other words, when these metals are mostly consumed for reduction of copper oxide, the action of these metals to catch oxygen present in the firing atmosphere, which is another effect of these metals, decreases, thus a satisfactory improvement in solderability is hard to be obtained.
There are known some proposals of utilization of borides in the electric and electronic fields.
For instance, U.S. Pat. No. 4,225,468 to Donohue discloses a resistor composition containing LaB.sub.6 and glass, which is fired in a nonoxidizing atmosphere. Also, French Patent No. 781400 discloses a resistor composition comprising a mixture of an alkaline earth metal aluminoborate glass frit and a metal hexaboride. These known compositions are for providing a resistor, and the patents do not disclose the improvement in solderability of thick film conductors by using copper with metal borides.
Metal silicides have heat resistance and oxidation resistance. They have also a higher conductivity than oxides and are usable as materials having a conductivity. Accordingly, silicides attract attention as a target in respect of semiconductors. However, in conventional copper conductor compositions, there is no proposal about the use of metal silicides.
It is proposed to use metal silicides in resistor compositions. For instance, U.S. Pat. No. 3,341,363 to Owen discloses a thick film resistor composition containing Si, WO.sub.3, MoSi.sub.2, Co, W, Mg and an inorganic powder binder consisting of kaolin and a glaze material. British Patent No. 1,559,523 discloses a resistor composition containing a borosilicate glass frit and a metal silicide. In electronic circuits, the metal silicides used in these resistor compositions function as a resistor rather than as a conductor. It is not taught at all to improve the solderability of thick film conductors by using copper with metal silicides.
It is an object of the present invention to improve the solderability of conventional copper conductor compositions.
A further object of the present invention is to provide a conductor composition capable of giving a thick film copper conductor having an improved solderability as well as other good properties such as adhesion to ceramic substrate and other substrates.
Another object of the present invention is to provide a conductor element comprising a ceramic substrate having a patterned copper conductor having an improved solderability as well as other good properties such as adhesion to the substrate.
These and other objects of the present invention will become apparent from the description hereinafter.